In my elder U.S. Pat. No. 3,255,706 I have disclosed a fluid pressure pocket in a piston wall to reduce the friction between the cylinder and the wall.
In said patent however, the piston carried a part-cylindrical pivot-portion of a piston shoe, whereby the piston shoe prevented a rotation of the piston around the axis of the piston.
In my grand-parental application, which is now U.S. Pat. No. 4,037,523, I have also disclosed a part-spherical pivot-member on a piston shoe, borne on a complementary formed bed on the piston. Such part-spherical pivot-portions are also common in the former art.
The part-spherical pivot portions can not prevent a rotation of the piston relatively to the piston shoe.
The piston however exert during pivotion of the piston shoe a lateral or tangential load onto the wall of the cylinder wherealong the piston slides. This load is in the direction of an imaginary plane constituted by the move of the imaginary axis of pivotion of the piston shoe.
Since the part-spherical pivot-portion of a piston shoe can not prevent a rotation of the piston relatively to the piston shoe, the fluid pressure pocket in the piston, when applied, might move out of the direction of said load and thereby prevent the desired carrying of the load by the fluid pressure pocket. It might even transfer the force in the fluid pressure pocket by rotating the piston to another, undesired direction. It would then press the piston on the opposite side against the cylinder wall and thereby increase the friction between the oscillating or reciprocating piston and the wall of the cylinder so drastically, that the device with the pocket would be of less efficiency than a device without a fluid pressure pocket in the piston.